Conventional propellant-based downhole stimulation employs only one ballistic option, in the form of a right circular cylinder of a single type of propellant grain, which may comprise a single volume or a plurality of propellant “sticks” in a housing and typically having an axially extending hole through the center of the propellant through which a detonation cord extends, although it has been known to wrap the detonation cord helically around the propellant grain. When deployed in a wellbore adjacent a producing formation, the detonation cord is initiated and gases from the burning propellant grain exit the housing at select locations, entering the producing formation. The pressurized gas may be employed to fracture a formation, to perforate the formation when spatially directed through apertures in the housing against the wellbore wall, or to clean existing fractures or perforations made by other techniques, in any of the foregoing cases increasing the effective surface area of producing formation material available for production of hydrocarbons or geothermal energy. In conventional propellant-based stimulation, due to the use of a single, homogeneous propellant and centalized propellant initiation, only a single ballistic trace in the form of a gas pressure pulse from propellant burn may be produced.
U.S. Pat. Nos. 7,565,930, 7,950,457 and 8,186,435 to Seekford et al., the disclosure of each of which is incorporated herein in its entirety by this reference, propose a technique to alter an initial surface area for propellant burning, but this technique cannot provide a full regime of potentially available and desirable ballistics (i.e., various solutions associated with pressure versus time possibilities resulting from propellant burn) for propellant-induced stimulation in a downhole environment. It would be desirable to provide enhanced control of not only the initial surface area (which alters the initial rise rate of the gas pulse, or dP/dt, responsive to propellant ignition), but also the duration and shape of the remainder of the pressure pulse introduced by the burning propellant.
U.S. patent application Ser. No. 13/781,217 by the inventors herein, filed Feb. 28, 2013, now U.S. Pat. No. 9,447,672, issued Sep. 20, 2016, assigned to the Assignee of the present disclosure and the disclosure of which has been previously incorporated herein by reference, addresses the issues noted above and left untouched by Seekford et al.
It is known to provide downhole structures configured for containing, at least in part, wellbore pressures elevated above hydrostatic for stimulation purposes. For example, U.S. Pat. No. 3,090,436 describes the use of opposing, cup-shaped packer members in a bottomhole assembly for containing pressurized fracturing fluid used for fracturing a formation intersected by a wellbore, the packer cups expanding. U.S. Pat. No. 3,602,304 describes the use of a propellant charge to set an anchor and packer above a propellant container housing propellant charges for fracturing. U.S. Pat. No. 7,487,827 describes the use of so-called “restrictor plugs” carried by a stimulation tool, which restrictor plugs project radially from a stimulation tool to restrict, but not prevent, flow of combustion gases generated by a propellant charge between the restrictor plugs and wellbore casing. U.S. Pat. No. 7,810,569 describes the use of expandable, high-pressure seals for containing elevated pressure used for fracturing a formation. U.S. Pat. No. 7,909,096 describes the use of packers and packer/bridge plug combinations for isolating pressure of a fluid used for stimulation. The disclosure of each of the foregoing patents listed in this paragraph is hereby incorporated herein in its entirety by reference.
The inventors herein have developed further enhancements to the methods and apparatus described in the '217 application, as described in U.S. patent application Ser. No. 14/491,518, filed Sep. 19, 2014, now U.S. Pat. No. 9,995,124, issued Jun. 12, 2018, the disclosure of which has also been previously incorporated herein by reference, as well as to the methods and apparatus described in the preceding paragraph. More specifically and with regard to the present disclosure, the inventors herein have developed apparatus incorporated into stimulations tools, and related methods, to enable more effective use of propellant-based stimulation tools producing relatively high, variable and extended duration pressure pulses, including, but not limited to, those described in U.S. patent application Ser. No. 13/781,217, filed Feb. 28, 2013, now U.S. Pat. No. 9,447,672, issued Sep. 20, 2016, and U.S. patent application Ser. No. 14/491,518, filed Sep. 19, 2014, now U.S. Pat. No. 9,995,124, issued Jun. 12, 2018.